Project Summary Microvesicles are submicron size, membrane enclosed fragments, released from cells in response to activation or during apoptosis. Platelet-derived microvesicles, released upon platelet activation, constitute a major fraction of microvesicles in the circulating blood. In addition to their well-known role in hemostasis, platelet microvesicles have been shown to stimulate hematopoietic cells, transfer platelet-specific receptors to the surface of other cells and elicit cytokine responses from synovial fibroblasts. In flowing blood, microvesicles are pushed towards the plasma-endothelial interface because of their size and they are poised to interact with endothelium. Interactions between platelets and endothelium have been known for a long time. Severe thrombocytopenia alters endothelial cell permeability and integrity. Purpuric hemorrhages in severe thrombocytopenia occur by rupture of endothelial cells rather than by extravasation through endothelial junction. Platelets derived growth factors regulate angiogenesis. Platelet microvesicles may mediate some of the effects attributed to platelets. The recent findings also suggest that platelet microvesicles induce angiogenesis and endothelial cell migration. We have recently shown that (developmental endothelial locus-1 (Del-1), a 52 kDa glycoprotein mediates the uptake of platelet microvesicles by endothelium both in vitro and in vivo. We propose that endothelium is the physiological mediator of microvesicles clearance. Uptake of microvesicles by the endothelial cells induces several functional and morphological changes in endothelium. More recently, we have shown that platelet microvesicles can deliver microRNAs endothelium raising the possibility that microvesicles can modulate endothelial cells function by modifying gene expression. We propose to characterize the significance of this novel finding. The specific aims of this proposal are (1) To further characterize the molecules involved in phosphatidylserine-dependent uptake and clearance of platelet microvesicles by endothelium (2) To determine the effect of microvesicles associated microRNAs on endothelial function and (3) to determine whether plasma levels of Del-1 and lactadherin are biomarkers of microvesicles clearance.